Discontinuous hollow cylindrical transducer



United States Patent O 3,246,288 DISCONTINUOUS HOLLOW CYLINDRICALTRANSDUCER Leon W. Camp, Santa Monica, Calif., assignor to BendixCorporation, North Hollywood, Calif., a corporation of Delaware FiledJune 2 1963, Ser. No. 290,950 5 Claims. (Cl. 34011) This inventionrelates to underwater transducers of the hollow cylindrical typevibrating in the radial mode, and the present application is acontinuation-in-part of my application Serial No. 205,952 filed June 28,1962.

An object of the invention is to improve the performance of transducersof this type as to band width and the ratio of the band width to theamount of active material.

Another object is to provide an eflicient underwater transducer withoutthe use of shielding or pressure release material.

Another object is to provide a highly efficient polarizedmagnetostriction transducer of this type in which permanent magnetssupply the polarizing field.

Other more specific objects and features of the invention will appearfrom the description to follow.

Longitudinally continuous (sometimes herein referred to as closed)radially vibratile hollow cylindrical transducers are known. It is alsoknown that in order to perform well, such transducers must not be tooshort, or the inside must be acoustically shielded from the outside toreduce interaction between the out-of-phase radiations from the insideand outside surfaces, respectively.

I have discovered that although, without auxiliary shielding, a certainoverall length must be maintained for good performance, the amount ofelectromechanicallysensitive material required can be substantiallyreduced by employing a plurality of short, axially-spaced cylinder orring elements instead of a continuous cylinder. It has been determinedthat the spacing between elements for good performance is between .1)\wand .75)\w-, where Aw is the wavelength of sound in the ambient liquidmedium, usually water. With a given amount of material, a plurality ofspaced cylinder elements has the following advantages over a single, orclosed, cylinder:

(a) It has a greater band width.

(b) It has a greater efficiency in converting electrical power intosound, and vice versa.

(c) It enables a desired high radiation impedance with a smallerquantity of material, which with a power source of given magnituderesults in a higher power intensity in the material, which is sometimesdesirable. Thus, a high power level is required to efficiently drive anunpolarized magnetostriction transducer.

The following detailed description refers to the drawing, in which:

FIG. 1 is a longitudinal sectional view of a transducer incorporatingthe invention.

FIG. 2 is an end view of the transducer shown in FIG. 1.

FIGS. 3 to 5 are graphs showing impedance characteristics of transducersin accordance with the prior art and the present invention,respectively.

FIG. 1 shown a transducer comprising four axiallyspaced cylindricaltransducer elements or rings Illa, 10b, 10c and 10d, all driven toexpand and contract circumferentially with resultant radial vibration.The elements may be of any known type insofar as the acoustic advantagesof axially-spaced rings are concerned. Thus they may be of the ceramicelement type, as disclosed in my Patent No. 2,733,423 or of themagnetostrictive type. Magnetostrictive transducers are usuallylaminated,

either by stacking flat annular laminations or winding 3 ,246,288 CPatented Apr. 12, 1966 a long strip into a tight scroll. At the moment,we are concerned with the advantages of axially spacing radiallyvibratile rings or cylinders, regardless of their material orconstruction.

In order to radiate sound energy into a fluid medium, a vibrating sourcemust experience a reaction from the medium, known as radiationimpedance. Absolute values of radiation impedance for short cylindersare difiicult to determine, but relative values are readily obtainedfrom impedance curves such as shown in FIGS. 3 and 4. Both curves werederived from transducers consisting of magnetostrictive rings 6" indiameter and of rectangular cross-section, measuring .3" radially andA2" axially. These rings were stacked in groups of three, each groupbeing enclosed by a winding and a neoprene housing. Such a group wouldnot show efiicient performance as a sound source, because theout-of-phase sonic pressures developed inside and outside would largelyneutralize each other in the ambient medium. However, a close assemblyof four such groups defines a longer closed wall between inside andoutside and becomes an efiicient source. The impedance curve of such afour-group assembly having an overall length of 7" is shown in FIG. 3.

An unexpected performance, shown by the curve of FIG. 4, is obtainedfrom an assembly of four groups identical with the above groups byspacing the groups apart axialy one-half wavelength (3.6" for thetransducer dimensions cited) of sound in the ambient medium (.SAw) atthe resonant frequency. FIG. 4 indicates a higher radiation impedancethan FIG. 3 because the impedance varies less with change of frequencynear the resonant frequency. Thus in FIG. 3, over a range of frequencyof 1 kc. from 7.75 to 8.75 kc., the resistance varies from about 45 ohmsto 72 ohms, or a range of 27 ohms, and the reactance varies between 204ohms and 190 ohms, or a range of 14 ohms. In contrast, in FIG. 4 oversubstantially the same 1 kc. range from 7.5 to 8.5 the resistance variesbetween about 45 ohms and 57 ohms, or a range of 12 ohms, and thereactance varies between about ohms and 172 ohms, or a range of 3 ohms.Contrariwise, within resistance and reactance ranges of 4 ohms, the openconstruction of FIG. 4 permits a frequency range of about 1.5 kc. asagainst less than .25 kc. for the closed construction.

The use of spaced rings or cylinders adds another degree of freedom inthe design of a transducer. Some illustrations of the advantages are asfollows:

(1) For the example shown, without changing the material used, anincrease in band width for the sound source was achieved; also itsoutput as a sound source per watt of input power was increased.

(2) One can obtain a given band width with approximately half the activematerial necessary in the closed construction.

(3) One can control the radiation impedance by spacing, while selectingthe appropriate quantity of active material to make possible thesolution of a problem in driving unpolarized magnetostrictive cylinders.The problem is that, without polarization, the active material can beefficiently driven only at a high power level. Therefore thepower-handling capability of the active material must be matched to theelectric power available.

Another example of the effect of proper spacing of short cylindricalrings on efficiency and band width is illustrated by FIG. 5, in whichCurve A shows the output of a single ring and Curve B shows the outputof four such rings. In each instance the length (in axial direction) ofthe individual rings is .104Aw. The four rings of Curve B were spacedapart .104kw. It will be noted from FIG. 5 that Curve A shows a bandwidth of only about 450 cycles lying within a range of four decibels aa) whereas in Curve B the band width within the same range is about 2000cycles.

The results of other tests are tabulated in the following table:

The one ring of Item (8) had a wall thickness (radial) of .3, threetimes that of the other rings which were .1". Comparing Item (8) withItem (4), both containing the same amount of material and having thesame band width, the better performance of Item (4) is evident.

Comparison of Items (7) and (6) shows the very great advantage of theaxially longer ring when used singly. Comparison of Items (7) and (3)shows the gain in using a plurality of spaced narrow rings over a singlering. However, Item (3) is still inferior to Item (2).

As a result of all my tests, it appears that an axial length ofapproximately .lAw gives optimum performance in terms of radiationefficiency both for single and multiple, spaced, rings, but it can be atleast as low as .OSAw and as great as .lSAw without extreme reduction inradiation efiiciency.

It appears that for the best radiation efiiciency, the optimum spacingin multiple ring transducers can vary between law, and .75 \w.

The total length of multiple ring arrays may vary over a wide rangedepending only on the desired beam width of the radiation pattern.

From the data, it appears that a radial width of .lkw is optimum bothfor single and multiple spaced elements; and also that spaces betweenelements in an array should not be much below .lAw or much greater than.Skw. Item (3) shows that, while not optimum, the use'of .05)\w widthmight be desirable at times where power output per pound is not theprimary consideration and low weight is important.

A further advantage of the invention is that it provides a latitude ofdesign to fit particular requirements. Thus as an example, a transducerhaving a resonant frequency of 350 cycles may be had with either of thefollowing specifications:

(1) Single ring of:

Mean diameter inches 134 Wall thickness -do 6.85 Length dO 23 Weight lbs20,800

(2) Two rings, spaced:

Overall length inches 113 Mean diameter do 103 Wall thickness do 1.77Length each do 17.7 Total weight lbs 6440 These two transducers, inspite of the disparity in weight, will perform identically except as tototal power capacity.

A hollow cylindrical ring vibrating radially in water (the liquid mediumin which devices in accordance with this invention are commonlyemployed) works against an impedance resulting from the mass reactanceof, and

the radiation of energy into, the Water. The mass reactance aflects thefrequency and band width of the resonance curve. The present inventionenables the control of the relative values of the mass reactance and theradiation resistance by properly choosing the spacing between the ringsand the overall length of the array of rings. The overall length is, ofcourse, a function of the spacing and the ring dimensions.

It has been known that very short cylindrical rings vibrating radiallyin water show a mass reactance high with respect to the radiationresistance, and that such rings are inefiicient converters of mechanicalenergy into acoustical energy.

The present invention greatly advances the art of transducer design inenabling the obtention of superior performance with the use of smalleramounts of active material and without the use of pressure release orother shielding material.

Where spaced magnetostrictive rings are used, as in the presentinvention, a highly desirable polarizing structure utilizing permanentmagnets becomes possible. This structure, as shown in FIG. 1, comprisesa set of uniformly circumferentially spaced permanent magnets 12:: and1211 between each adjacent pair of rings 10a and 10b, 10b and 100, 10cand ltld, respectively. Each set of magnets includes equal numbers ofmagnets 12a and 121], respectively, alternately spaced; The onlydifference between magnets 12a and 12b is that they are oppositelypoled.In FIG. 1 all magnets 12a have their south poles uppermost, and allmagnets 12b have their north poles uppermost. This produces acircumferential flux in each quadrant of each ring, the direction ofwhich flux is opposite to that in adjacent rings. Thus the relativedirections of the fluxes in different quadrants of the top ring in FIG.1 are shown by the arrows in FIG. 2. In corresponding quadrants of eachpair of adjacent rings, the direction of the polarizing flux fromquadrant to quadrant, the directions of the winding 14 on each ring mustalso reverse from quadrant to quadrant, as shown in FIGS. 1 and 2, inorder for the effects of the current in different quadrants to be inadditive instead of subtractive relation to each other.

FIG. 1 shows a body 15 inserted in series with each magnet 12a or 1212.These bodies are of high permeability material to keep the reluctance ofthe magnetic circuits low. In some instances the bodies 15 may beeliminated, and the permanent magnets 12 dimensioned to fill the gapsbetween adjacent rings 10. However, many otherwise desirable permanentmagnet materials have a very low permeability, and it is desirable tokeep the permanent magnets as short as possible and fill the gap with ahighly permeable material, such as powdered or laminated soft iron orone of the many alloys having even higher permeability.

Although for the purpose of explaining the invention a particularembodiment thereof has been shown and described, obvious modificationswill occur to a person skilled in the art, and I do not desire to belimited to the exact details shown and described.

I claim:

1. A transducer for immersion in a liquid medium for translating soundwaves in said medium into electrical waves in an electric circuit, andvice versa, comprising:

a plurality of axially-aligned, spaced-apart,electrornechanically-responsive, radially-vibratile rings and meanselectrically coupling said rings to said circuit for vibration in phasewith each other;

said transducer being open at the ends and between said rings wherebysubstantially all sides of each ring are in direct acousticcommunication with a liquid in which said transducer is immersed.

2. Apparatus in accordance with claim 1 in which said rings are axiallyspaced apart less than .75)\w where Aw is the wavelength in said liquidmedium at the opening frequency of said rings.

References Cited by the Examiner UNITED STATES PATENTS 3,160,769 12/1964Abbot 34011 CHESTER L. J USTUS, Primary Examiner.

G. M. FISHER, Assistant Examiner.

1. A TRANSDUCER FOR IMMERSION IN A LIQUID MEDIUM FOR TRANSLATING SOUNDWAVES IN SAID MEDIUM INTO ELECTRICAL WAVES IN AN ELECTRIC CIRCUIT, ANDVICE VERSA, COMPRISING: A PLURALITY OF AXIALLY-ALIGNED, SPACED-APART,ELECTROMECHANICALLY-RESPONSIVE, RADIALLY-VIBRATILE RINGS AND MEANSELECTRICALLY COUPLING SAID RINGS TO SAID CIRCUIT FOR VIBRATION IN PHASEWITH EACH OTHER; SAID TRANSDUCER BEING OPEN AT THE ENDS AND BETWEEN SAIDRINGS WHEREBY SUBSTANTIALLY ALL SIDES OF EACH RING ARE IN DIRECTACOUSTIC COMMUNICATION WITH A LIQUID IN WHICH SAID TRANSDUCER ISIMMERSED.